1. Field of the Invention
The present invention is related to electrical energy measuring devices for electric utility systems, and more particularly to solid state devices utilizing digital processing techniques.
2. Description of the Prior Art
The electromechanical rotating disc type of watthour meter continues to enjoy almost exclusive use in electrical metering applications. However, because of the desire to effectuate other services such as remote meter reading, time of day metering, and load control substantial time has been invested in developing cost effective alternatives to the standard watthour meter. One such line of products from this development is the solid state microprocessor based electric energy metering system.
With the advent of low cost solid state circuits such as microprocessors, programmable read only memories (PROM's), random access memories (RAM's), etc., solid state electric energy metering systems are becoming increasingly more common. Nevertheless, substantial problems have to be overcome before solid state circuits can be applied to electric energy metering systems. One problem is due to the fact that solid state circuits operate utilizing low voltage input signals whereas the input signals available at the point of measurement for metering systems are line voltage and line current. Substantial problems are encountered in producing the requisite low level input signals from line voltage and line current. Another problem is that line current varies from approximately 1/2 to 200 amperes. Still another problem is that devices such as microprocessors operate digitally. This means that line voltage and current, or signals representative thereof, must be digitized with the attendant problems of sampling rates and sample resolution. Also, a microprocessor needs a list of instructions, or a program, in order to properly run the solid state metering system. Very often transients, noise, or unexpected disturbances will disrupt the microprocessor causing the metering system to malfunction. These and other problems have been solved to some degree by currently available solid state electric energy metering systems.
An example of one such metering system is disclosed in U.S. Pat. No. 4,077,061. Disclosed therein is a digital processing and calculating AC electric energy measuring system which includes a sequence controller and a calculator subsystem for controlling the metering system operations in accordance with a predetermined program. An analog input circuit receives voltage and current signal components of an electric energy quantity to be measured by the metering system. A sample timer circuit utilizes the sequence controller and calculator subsystem clock for producing sample interval timer pulses which initiate each randomized sampling interval. Instantaneous sample values of the voltage and current signals are thus obtained at randomized sampling times in response to the sample timer interval pulses. The instantaneous signal values are sequentially digitized by an analog-to-digital converter. From this raw data a plurality of parameters of the electric energy quantity to be measured are calculated in a common calculation program subroutine operating on the digitized instantaneous signal values. Totalizing each of these calculated values produces the value of the time integral of the measured electric energy parameter. An output readout or display produces numerical readings representative of the plurality of electric energy parameters calculated by the metering system. A plurality of output pulse data signals representing the calculated parameters are input to a pulse receiver device capable of transmitting the data pulses through a remote metering telemetry system or for being recorded in a recorder type of receiver device.
While the system described above, as well as several other types of electric energy metering systems, has overcome the basic problems of applying solid state technology to metering applications, there nevertheless remains substantial room for improvement, particularly in the areas of accuracy and reliability. The present invention is for an AC electric energy metering system having substantial improvements in the accuracy and reliability of the system.